Sprinkle const around where appropriate (#1909)

* Sprinkle `const` around where appropriate

- This will make it easier to use `NDS` objects in `const` contexts (e.g. `const` parameters or methods)

* Remove the `const` qualifier on `DSi_DSP::DSPRead16`

- MMIO reads can be non-pure, so this may not be `const` in the future

src/DSi_NAND.cpp

@@ -365,7 +365,7 @@ bool NANDImage::ESEncrypt(u8* data, u32 len) const
     return true;
 }
 
-bool NANDImage::ESDecrypt(u8* data, u32 len)
+bool NANDImage::ESDecrypt(u8* data, u32 len) const
 {
     AES_ctx ctx;
     u8 iv[16];